High-voltage and extra-high-voltage
(EHV) transmission lines interconnect power plants and loads, and
form an electric network. This system contains 500-kV, 345-kV,
230-kV, and 115-kV lines. T

Presently, synchronous ties (AC lines)
interconnect all networks in the eastern U.S. and Canada. Synchronous
ties also (AC lines) interconnect all networks in the western U.S.
and Canada. Several nonsynchronous ties (DC lines) connect the East
and the West. These interconnections increase the reliability of the
electric supply systems.

In the U.S., the nominal voltage of the
high-voltage lines is between 100 kV and 230 kV. The voltage of the
extra-high-voltage lines is above 230 kV and below 800 kV.

The voltage of an ultra-high-voltage
line is above 800 kV. The maximum length of high-voltage lines is
around 200 miles.

Extra-high-voltage transmission lines
generally supply energy up to 400–500 miles without intermediate
switching and var support. Transmission lines are terminated at the
bus of a substation.

The physical arrangement of most
extra-high-voltage (EHV) lines is similar. The figure below shows the major components of an EHV, which
are:

At lower voltages the appearance of
lines can be improved by using more aesthetically pleasing steel
tubular towers. Steel tubular towers are made out of a tapered steel
tube equipped with banded arms.

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